 Synthesis of Dichloromethyl methyl ether
In a fume cupboard equip a 500-ml three-necked flask with a dropping funnel, a mechanical stirrer, a thermometer and a reflux condenser, using a double-neck adapter (Fig. 2.15) to accommodate the last two items. Protect the condenser and dropping funnel with calcium chloride guard-tubes. Place 50 ml of phosphorus oxychloride and 156 g (0.75 mol) of phosphorus pentachloride in the flask and 48 g (49 ml, 0.8 mol) of methyl formate in the dropping funnel. Cool the mixture to 10°C and add the methyl formate dropwise at such a rate that the reaction temperature does not rise above 20°C (about 1 hour). When the addition is complete, remove the ice-bath, stir the mixture until all the phosphorus pentachloride has dissolved, keeping the temperature below 30°C by occasional cooling. Remove the stirrer, condenser, thermometer and dropping funnel, stopper the two side-necks and insert a simple distillation head into the central neck. Introduce a pine splint to serve as anti-bumping device (*) and attach a condenser leading to a receiver flask a receiver adapter for vacuum distillation. Distil the reaction mixture under reduced pressure (water pump) on a water bath at about 60°C and collect distillate in a flask cooled to -25 to -30°C in a cooling bath (acetone-Cardice or ice-salt). The distillate weighs 209 g and consists of a mixture of dichloromethyl methyl ether (b.p. 85°C) and phosphorus oxychloride (b.p 105°C). Fractionally distil the mixture at atmospheric pressure through a 50-cm column filled with glass helices and surrounded by a heating jacket at 60°C, using a reflux ratio of about 1:8. Collect the fraction which boils between 82 and 95°C and refractionate to give 70g (76%) pure dichloromethyl methyl ether, b.p. 85°C. Protect the product from moisture. (*) A conventional capillary air leak, inserted into a side-neck, should only be used in this case if the air supply is pre-dried since the product is susceptible to decomposition by moisture. Reference: Vogel, Practical Organic Chemistry, 5th Ed. p 582-583 |